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# BJT biasing formulas

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total Z seen by the MIC (Zmic)= 10K//11.2K= 5.3K

Correct.

C1 (=330n) and Zmic are a high pass filter, its cut off freq (Fo)
Fo= 1/(2 pi C1 Zmic)= 91 Hz

No. The mic impedance is about 5k and it is in parallel with its 10k load/powering resistor so it has an effective series resistance of 3.3k. The 3.3k, the 330nF coupling capacitor and the 11.2k load resistance are all in series so the cutoff frequency is 1/(2 pi C1 [3.3k + 11.2K])= 33Hz.

Fo = 64 Hz, so this filter cuts all freqs less than 64Hz including mains hum.

No. This is a very simple filter. At the cutoff frequency the response is down -3dB which is a small amount. If the cutoff frequency is 500Hz then 50Hz would be reduced but not eliminated.

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Good evening Guru
Thank u for fast and good reply

This is a very simple filter. At the cutoff frequency the response is down -3dB which is a small amount. If the cutoff frequency is 500Hz then 50Hz would be reduced but not eliminated.

Now I'll not ask you about filters and the -3dB, I'll postpone this to another discussion, but I noticed that C1 and the other Rs are not intended as a filter.
You choosed C1 as the smallest possible cap  to couple the two parts of the circuit, and not els.

Only at high frequencies and if there is no load.

At high frequencies, to what range?
To a freq make Xc4 = R5, if so, to f=2258Hz a human voice.

At 15kHz, the reactance of the 150nF C4 capacitor
is 1070 ohms in parallel with R5= 71.1 ohms.

Xc4 at 15KHz = 70.7 ohm in parallel with R5= 61.5 ohms, not so far.
I know this is not problem, and this occur whe somebody do fast calculation.

Therefore at 15kHz the voltage gain is about 0.707 x 10k/(71.1 + 86.70= 63.4.

Two questions:
(1) why u choose the 15KHz personally?
(2) I understand [10k/(71.1 + 86.70)] but not 0.707
I know that 1/sqr(2) = 0.707
but why multiplying the Av by it, is it a known formula?

At 300Hz, the reactance of ....

this statement answer the #1 question above, 300Hz is the lower limit of the human voice and the 15KHz is the upper limit.

At 300Hz, the reactance of the 150nF C4 capacitor is 53.5k so it doesn't affect the gain. Then the voltage gain is only 10k/(470 + 86.7)= 18. So the treble boost at 15kHz is about 3.5 times.

The idea is very clear and you provide me with a very good and new information, but to others who may read our discussion, I tell them GURU has some headache because he answring hunreds of questions every day, GOD save guru from any bad thing.

At 300Hz, the reactance of the 150nF C4 capacitor is 3.5k
3500//470 = 414 ohm
Then the voltage gain is only 10k/(414 + 86.7)= 20. So the treble boost at 15kHz is about 3.2 times. NO DIFFERENCE

GURU you are good man and "Electronics Lab - Community = GURU"
Thank u for every word you write to help me.

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You choosed C1 as the smallest possible cap

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• 4 months later...

Hi
I'll back to the first design; C_E without CE.
I think in the past that increasing the value of Ic have better property to get higher o/p power,
and I was prefering -according to this- to design circuits with higher Ic values
GURU explained that this was a mistake and from my discussions with him I found that the
1-consumption of the battery rapidly
2- more Ic = less hfe
My questions :
(1) Are there any other disadvantages?
(2) when I have to choose higher values for Ic?

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when I have to choose higher values for Ic?

1) When the load requires a higher current.
2) When the output capacitance of the transistor and stray capacitance combines with the collector resistor's value to form a lowpass filter. A lower value of the resistor would require a higher collector current for a higher frequency response.
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2) When the output capacitance of the transistor and stray capacitance combines with the collector resistor's value to form a lowpass filter. A lower value of the resistor would require a higher collector current for a higher frequency response.

Please I want further clarification on this point, I hope to support that with examples and illustrations
thanks
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A transistor's output impedance is the value of its collector resistor. The collector resistor is the signal source and the capacitance of the transistor's collector plus stray capacitance forms a lowpass filter with it.
If the value of the collector resistor is lower then the collector current would need to be increased but the lower resistor value makes the frequency higher of the lowpass filter.

I have forgotten all the high frequency transistor formulas that I learned more than 40 years ago and have never used. Maybe you can find examples and illustrations on the internet.

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Thank you guru, I think that this is a big subject
it is better to focus now on topics simple and and when finished turn to the more difficult topics.

I want to tell you that a great honor for me to debate with you, you great wealth of scientific.

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